Don't ignore v2s1 history if docker_version is not set
Don't ignore v2s1 history if docker_version is not set

buildah itself clears docker_version in Builder.fixupConfig; so,
buildah-created v2s1 images do not pass this test
(buildah from buildah-created-v2s1) creates an image with empty
history, which causes (buildah commit) to complain about the mismatch
between history and layer DiffID list.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

Closes: #776
Approved by: rhatdan

TomSweeneyRedHat/buildah,projectatomic/buildah,projectatomic/buildah,nalind/buildah,ipbabble/buildah,TomSweeneyRedHat/buildah,rhatdan/buildah,projectatomic/buildah,TomSweeneyRedHat/buildah,nalind/buildah-1,nalind/buildah-1,nalind/buildah,nalind/buildah-1,rhatdan/buildah,rhatdan/buildah,ipbabble/buildah